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How does the size of the rear derailleur affect performance in shifting?
I have heard that a longer cage derailleur takes longer to shift and might not do it as smooth as a short cage. But how big is this difference really? If I need only a mid cage, is it stupid then to buy a long cage?

Bikes: Catrike 559 I own some others but they don't get ridden very much.

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I only have extensive experience with Shimano stuff so my observations might not apply to Campy.

As near as I can tell, the parallelgrams of Shimano rear derailleurs, which is the part that determines shifting quickness, are the same. The only thing that the longer derailleur cage does is to wrap up more chain slack. If you have wide range gearing, you need the greater slack take up capability of the long cage. The short cage derailleur, of course, looks racier. Otherwise, I don't see much advantage in one over the other.

Retro is correct. Also a longer cage vs a shorter cage is not slower, what's slower is the larger spread out (touring or MTB) gears the longer cage is designed to handle, vs the shorter less range (road) gears a shorter cage is designed for. But Shimano XTR does an amazing quick job of shifting larger gear ranges. If you have a typical road bike then you have typical close range gearing and the shorter cage is idea for that.